The present invention relates to a pattern formation method for use in fabrication process or the like for semiconductor devices.
In accordance with the increased degree of integration of semiconductor integrated circuits and downsizing of semiconductor devices, there are increasing demands for further rapid development of lithography technique. Currently, pattern formation is carried out through photolithography using exposing light of a mercury lamp, KrF excimer laser, ArF excimer laser or the like, and use of F2 laser lasing at a shorter wavelength of 157 nm is being examined. However, since there remain a large number of problems in exposure systems and resist materials, photolithography using exposing light of a shorter wavelength has not been put to practical use.
In these circumstances, immersion lithography has been recently proposed for realizing further refinement of patterns by using conventional exposing light (for example, see M. Switkes and M. Rothschild, “Immersion lithography at 157 nm”, J. Vac. Sci. Technol., Vol. B19, p. 2353 (2001)).
In the immersion lithography, a region in an exposure system sandwiched between a projection lens and a resist film formed on a wafer is filled with a liquid having a refractive index n (whereas n>1) and therefore, the NA (numerical aperture) of the exposure system has a value n·NA. As a result, the resolution of the resist film can be improved.
Also, in order to increase the refractive index, use of an acidic solution as the immersion liquid has been proposed (see, for example, B. W. Smith, A. Bourov, Y. Fan, L. Zavyalova, N. Lafferty, F. Cropanese, “Approaching the numerical aperture of water—Immersion Lithography at 193 nm”, Proc. SPIE, Vol. 5377, p. 273 (2004)).
Now, a conventional pattern formation method employing the immersion lithography will be described with reference to FIGS. 9A through 9D, 10A and 10B.
First, a positive chemically amplified resist material having the following composition is prepared:
Base polymer: poly((norbornene-5-methylene-t-butylcarboxylate) (50 mol %)-(maleic anhydride) (50 mol %)) . . . 2 g
Acid generator: triphenylsulfonium trifluoromethane sulfonate . . . 0.04 g
Quencher: triethanolamine . . . 0.002 g
Solvent: propylene glycol monomethyl ether acetate . . . 20 g
Next, as shown in FIG. 9A, the aforementioned chemically amplified resist material is applied on a substrate 1 so as to form a resist film 2 with a thickness of 0.35 μm.
Then, as shown in FIG. 9B, a barrier film 3 having a thickness of 50 nm and made of an alkali-soluble barrier film material having the following composition is formed on the resist film 2 by, for example, spin coating:
Base polymer: polyvinyl alcohol . . . 1 g
Solvent: n-propyl alcohol . . . 25 g
Next, as shown in FIG. 9C, the resultant barrier film 3 is baked with a hot plate at a temperature of 120° C. for 90 seconds.
Then, as shown in FIG. 9D, with water 4, that is, an immersion liquid, provided on the barrier film 3, pattern exposure is carried out by irradiating the resist film 2 through the water 4 and the barrier film 3 with exposing light 5 of ArF excimer laser having NA of 0.68 having passed through a mask 6.
After the pattern exposure, as shown in FIG. 10A, the resist film 2 is baked with a hot plate at a temperature of 105° C. for 60 seconds, and thereafter, the resultant resist film 2 is developed with a 2.38 wt % tetramethylammonium hydroxide developer. In this manner, a resist pattern 2a made of an unexposed portion of the resist film 2 and having a line width of 0.09 μm is formed as shown in FIG. 10B.
However, as shown in FIG. 10B, the resist pattern 2a obtained by the conventional pattern formation method is in a defective shape. Furthermore, residues 2b are produced.